1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED), and more particularly, to a method for driving the OLED and related OLED driving circuit.
2. Description of the Prior Art
Having a variety of advantages, such as high light intensity, high response velocity, wide viewing angle, spontaneous light source and thin appearance, an organic light emitting diode (OLED) is becoming one of the most popular light emitting components that form a display device.
An OLED is a current-driving component. That is, the intensity of light (gray scale) emitted by an OLED can be controlled by determining currents flowing through the OLED.
A method for controlling the intensity of light emitted by an OLED by adjusting levels of currents flowing through the OLED is to adjust a voltage at a gate of a thin film transistor (TFT) serially connected to the OLED to control the levels of currents flowing through the OLED and to control the intensity of light emitted by the OLED. The TFT and the OLED combine to form an active display cell. The larger a voltage difference between the gate and a source of the TFT is, the greater the currents flowing through the OLED are and the larger the gray scale that the OLED performs becomes, and vice versa.
In the process that the TFT drives the OLED, not only the quality of the OLED dominates the performance of images displayed by the active display cell, but also how stable a threshold voltage of a transistor used to drive the TFT can be sustained is a key factor in determining whether the active display cell can display for a long enough period of time or not. Please refer to FIG. 1, which is a circuit diagram of an active display cell 10 according to the prior art. The cell 10 comprises a PMOS transistor T1 and an OLED 80 serially connected to the PMOS T1. A source, a gate and a drain of the PMOS T1 are connected to a first voltage source Vdd, a control voltage source VC and an anode of the OLED 80 respectively. A cathode of the OLED 80 is connected to a second voltage source VSS.
When a voltage generated by the control voltage VC is too small to turn on the PMOS T1 the PMOS T1 does not actuate any currents and the OLED 80 serially connected to the PMOS T1 does not emit light either. On the contrary, when the control voltage source VC generates a voltage that is large enough to turn on the PMOS T1, the PMOS T1 is turned on and actuates its currents capable of enabling the OLED 80 to emit light. Since the OLED 80 is an electronic component meant for emitting light, the PMOS T1 flows all the time the currents are capable of driving the OLED 80 to emit light. Whenever the PMOS T1 has currents flowing through, current carriers (holes for PMOS) are to flow along a direction directed by a first electric field E1 all the way from the source to the drain of the PMOS T1, and some current carriers may accumulate at a region between the source and the drain of the PMOS T1, resulting in a decrease of a threshold voltage Vthp of the PMOS T1.
Please refer to an equation 1, ld p=K(Vgs p+Vth p)2, which is a relation of a current Idp flowing through the PMOS T1 and a difference between a voltage difference Vgsp between the gate and the source of the PMOS T1 and the threshold voltage Vthp of the PMOS T1. It can be seen from the equation 1 that when the voltage difference Vgsp is kept constant, the current Idp flowing through the PMOS T1 drops as the threshold voltage Vthp of the PMOS T1 decreases. Therefore, currents flowing through the PMOS T1 controlled by a constant voltage, voltage difference Vgsp between the date and the source of the PMOS T1, will diminish as time goes by and the OLED 80 can only emit dimmer and dimmer light.
In FIG. 1, what the active display cell 10 utilizes to control the OLED 80 to emit light is the PMOS T1. However, the active display cell 10 can comprise an NMOS to control operations of the OLED 80 instead. Please refer to FIG. 2, which is a circuit diagram of a second active display cell 20 according to the prior art. The cell 20 comprises an NMOS T2 and an OLED 82 serially connected to the NOMS T2. A source, a gate and a drain of the NMOS T2 are connected to a second voltage source VSS, the control voltage source VC and a cathode of the OLED 82. An anode of the OLED 82 is connected to the first voltage source Vdd.
When the control voltage source VC generates a voltage to turn off the NMOS T2, the NMOS T2 does not generate any currents and the OLED 82 serially connected to the NMOS T2 does not emit any light either. On the contrary, when a voltage that the control voltage source VC generates is large enough to turn on the NMOS T2, the NMOS T2 will actuate currents capable of enabling the OLED 82 to emit light. Whenever the NMOS T2 has currents flowing through, current carriers (electron for NMOS) will flow along a direction opposite to a direction directed by a second electron field E2 all the way from the source to the drain of the NMOS T2, and some of the current carriers may accumulate at a region between the source and the gate of the NMOS T2, resulting in an increase of a threshold voltage Vthn of the NMOS T2.
Please refer to an equation 2, Id n=K(Vgs n−Vth n)2, which shows a relation between a current Idn flowing through the NMOS T2 and a difference between a voltage difference Vgsn between the gate and the source of the NMOS T2 and a threshold voltage Vthn of the NMOS T2. The equation 2 shows that when the voltage difference Vgsn is kept constant, the current Idn drops as the threshold voltage Vthn increases. Therefore, currents flowing through the NMOS T2 controlled by a constant voltage, voltage difference Vgsn between the date and the source of the NMOS T2, will diminish as time goes by and the OLED 82 can only emit dimmer and dimmer light.